The present invention relates to methods and a machine for determining a shape contour on a measurement object. More particularly, the invention relates to a new approach for determining a shape contour on a measurement object by using a coordinate measuring machine.
Coordinate measuring machines are flexible measuring devices which are primarily used in industrial production measurement technology. They usually have a measuring head which can be moved relative to a workpiece holder within a measurement volume. The measurement volume is spanned by the movement axes along which the measuring head can move relative to the workpiece holder. For the measurement, the measuring head is brought into a defined position relative to various selected measuring points. Spatial coordinates for each selected measuring point can then be determined from the position of the measuring head within the measurement volume and from the position of the measuring head relative to the selected measuring points. The spatial coordinates define the location of the measuring point within the measurement volume. If the spatial coordinates are determined for a multiplicity of measuring points, geometric dimensions on the object to be measured can be determined by using the spatial coordinates.
Originally, coordinate measuring machines were primarily used for individual point measurements. For this purpose, the measuring head has a stylus with a free end, which is brought into contact with the selected measuring point. Simple measuring heads are only able to produce a switching signal, which signals the contact with the selected measuring point. So-called scanning or gauging measuring heads are furthermore able to determine a deflection of the stylus relative to the measuring head during sensing. With gauging measuring heads it is possible to travel along the contour of a workpiece in a so-called scanning mode, in order in this way to determine a multiplicity of measured values and consequently a multiplicity of spatial coordinates for a multiplicity of measuring points along the contour.
Moreover, there are nowadays various non-contacting measuring heads for coordinate measuring machines. To be mentioned by way of example are measuring heads with capacitive sensors or measuring heads which, by means of laser triangulation, laser interferometry or with the aid of focal measuring methods, optically determine a distance between the measuring head and selected measuring points on the object to be measured.
For highly precise shape measurements, such as the roundness measurement on rotational bodies, until now specific shape measuring devices have generally been used. These are substantially less flexible than classic coordinate measuring machines. They are optimized to the desired shape measurement on specific objects to be measured, at the cost of flexibility.
For instance, DE 195 22 276 C1 discloses a shape measuring device for measuring external taper threads. The object to be measured, having an external taper thread, is clamped on a carriage between two points such that it can rotate. The carriage is then aligned in such a way that the axis of the external thread and the measurement axis of a feeler that can be moved in precisely one coordinate direction are located orthogonally relative to each other. With the aid of measuring wires, which are laid in mutually opposite thread gaps and are sensed by the feeler, it is possible to record measured values from which the flank diameter and the taper angle of the object to be measured are determined. Following the alignment of the object to be measured, the carriage can be locked.
DE 10 2008 058 198 A1 describes a flexible coordinate measuring machine of portal design. The measuring head is arranged at the lower end of a quill, which can be moved in the vertical direction. The quill is arranged on a carriage, which can be moved in a first horizontal direction. The carriage is arranged on a portal, which can be moved in a second horizontal direction. The portal is mounted on a base, on which there is arranged a workpiece holder in the form of a rotary table. The rotary table makes it possible to rotate a measurement object relative to the measuring head. In principle, a shape measurement is therefore possible. For instance, a rotationally symmetrical object to be measured can be rotated by the rotary table while the measuring head scans the outer circumference of the object to be measured.
The measuring accuracy of such a shape measurement is limited, however, as compared with specific shape or roundness measuring devices, although DE 10 2008 058 198 A1 suggests, inter alia, determining the inclination of the rotary table relative to the base as a result of an eccentric loading and correcting the resultant wobble error. Despite this correction, there is the desire to further increase the measuring accuracy of such a coordinate measuring machine with rotary table during shape measurements.
CN 201059953 Y discloses a manually operated coordinate measuring machine of portal design, in which the operator has a keypad in the area of the measuring head, with which he is able to selectively activate or deactivate the air supply into the air bearings of the individual axes. It is therefore possible to block individual movement axes of the coordinate measuring machine.
Furthermore, it is known from EP 0 701 663 B1 or DE 10 2011 112 734 A1 to deactivate air bearings of a coordinate measuring machine when the coordinate measuring machine stops for a certain time, in order in this way to save power. DE 10 2010 006 297 A1 proposes, in such a case, to keep the position controller operating even when the air bearings are deactivated, in order to minimize initialization steps when resuming operation.
From DE 196 23 601 A1 or EP 1 862 761 B1, it is known to block the stylus of a tactile measuring head in one or two coordinate directions, in order to permit a deflection of the stylus only in the coordinate direction or directions that remain.
From EP 0 402 440 B1, it is known to scan the surface of a measurement object continuously with a stylus while the measuring head is stationary. In this case, the measuring head has a two-axis rotary swivel joint, with the aid of which the stylus can be rotated relative to the measuring head about two orthogonal axes.